1. Field of the Invention
The present invention relates to an apparatus for detecting a rotating speed of a rotating object such as a rotating motor.
2. Description of the Prior Art
In a conventional apparatus for detecting a rotating speed of a motor, a pulse train having a frequency proportional to the rotating speed is generated and the rotating speed is detected by measuring the period of the pulse train at occurrence of each pulse.
Generally, when a motor rotating speed is to be controlled, the rotating speed detecting apparatus is designed so that the phase delay thereof becomes at most 30 degrees in the controlled frequency range to insure the stability of the control system. Due to this requirement, the control frequency range of the rotation control system is limited to a range which is 1/12 of the frequency of the pulse train. In other words, the response speed of the system cannot be increased.